The present invention relates to a phase shift oscillator and in particular to a phase shift oscillator formed with a surface acoustic wave coupled resonator having at least a 180.degree. phase shift occurring between the 3 dB points of the frequency response curve of the resonator. In particular the invention relates to a surface acoustic wave coupled resonator phase shift oscillator which oscillates at any desired frequency, either VHF or UHF, does not require inductors or capacitors, is relatively inexpensive, is shock insensitive and is temperature sensitive only as determined by the surface acoustic wave resonator.
Oscillators formed from surface acoustic wave devices are well-known in the art such as those disclosed in U.S. Pat. No. 3,868,595 and 3,950,713. These surface acoustic wave oscillators are formed from surface wave delay line devices used as the feed back network in the oscillator circuit. The surface acoustic wave delay line oscillator disclosed in U.S. Pat. No. 3,950,713 is for operation at relatively low frequencies since at high frequencies reflection problems become so pronounced that alternative techniques must be used. U.S. Pat. No. 3,868,595 also discloses a surface wave delay line oscillator operating at UHF frequencies. However, in order to provide stable oscillations, the input and output of the amplifier must be coupled to the output and input of the surface acoustic wave device through matching and/or phase shifting networks including inductors, capacitors and resistors.
The trend in the technology, of course, is to make the devices much less expensive, very small and compact. Such devices can be used in applications such as garage door opener transmitters, security devices, local oscillators and the like. In addition, military applications for stable oscillators are numerous. Military oscillators must be shock insensitive and temperature insensitive. Oscillators such as that disclosed in U.S. Pat. No. 3,950,713 are relatively shock insensitive and temperature insensitive but cannot operate at high frequencies. The surface acoustic wave oscillator disclosed in U.S. Pat. No. 3,868,595 can be used at high frequencies but is shock sensitive as well as temperature sensitive since the coils and capacitors vary the phase shift and the resultant frequency and the vibration of these devices or their change in physical shape because of temperature change respectively causes phase noise and changes the frequency of the oscillator.
The present invention overcomes the difficulties of the prior art by allowing the construction of a surface acoustic wave oscillator using coupled resonators having a 180.degree. phase shift occurring between the 3 dB points of the frequency response curve thus allowing the oscillator to operate at any design frequency without the use of inductors or capacitors and which is shock insensitive and temperature insensitive.
SAW coupled resonators are well-known in the art such as that disclosed in U.S. Pat. No. 3,970,970. A coupled resonator consists of two SAW resonators acoustically coupled to one another and yielding a to pole narrow band pass response. In addition, betweer the 3 dB points of the frequency response curve a phase shift of 180.degree. occurs. Thus with a phase shift of 180.degree. at the center frequency the phase shift can vary from 90.degree. at the lower 3 dB point to 270.degree. at the higher 3 dB point. Simply by reversing the bonding or changing the signal connection to the input pads of the SAW device, the phase shift at the center frequency is shifted to 0.degree. and thus covers the range from 270.degree. to 90.degree.. Thus the entire 360.degree. spectrum is covered with the coupled resonator oscillator.
Therefore, it is an object of the present invention to provide a phase shift oscillator having an amplifier with an input and an output and a surface acoustic wave device having an input and an output, a frequency response passing through 3 dB points and at least a 180.degree. phase shift occurring between the 3 dB points of the frequency response curve and means for connecting the output of the amplifier to the input of the SAW device and the input of the amplifier to the output of the SAW device to form an inductorless phase shift oscillator.
It is also an object of the present invention to provide a phase shift oscillator which utilizes a coupled resonator as the surface acoustic wave device coupled to an amplifier and having at least a 180.degree. phase shift occurring between the 3 dB points of the frequency response curve of the coupled resonator.
It is still another object of the present invention to provide a phase shift oscillator having a transistor as an amplifier which is coupled to a surface acoustic wave device formed as a coupled resonator and having at least a 180.degree. phase shift occurring between the 3 dB points of its frequency response curve.
It is yet another object of the present invention to provide a phase shift oscillator which utilizes a coupled resonator formed with a single phase unidirectional transducer and having at least a 180.degree. phase shift occurring between the 3 dB points of its frequency response curve.
It is also an object of the present invention to provide a phase shift oscillator which utilizes a surface acoustic wave resonator structure having a piezoelectric substrate, first and second spaced gratings of .lambda./4 electrodes attached to the substrate, input and output spaced transducer structures having .lambda./4 electrodes and attached to the substrate between the first and second gratings for generating standing waves in the substrate and a resonant cavity formed on the substrate between one of the gratings and one of the transducers.